Problem: Which of these numbers is prime? ${11,\ 14,\ 33,\ 49,\ 91}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 11 are 1 and 11. The factors of 14 are 1, 2, 7, and 14. The factors of 33 are 1, 3, 11, and 33. The factors of 49 are 1, 7, and 49. The factors of 91 are 1, 7, 13, and 91. Thus, 11 is a prime number.